1. Field of the Invention
The present invention relates to a light emitting diode (LED) having a vertical structure, and more particularly, to an LED having a vertical structure for improving light emitting efficiency and a method for fabricating the same.
2. Discussion of the Related Art
Light emitting diodes (LEDs) are well-known semiconductor light-emitting devices which convert electric current into light energy. The LEDs have been used as light sources for display images of electronic equipment including information communication equipment, in conjunction with green LEDs using GaP:N semiconductors, starting from the commercialization of red LEDs using GaAsP semiconductors in 1962.
The wavelength of light emitted by such LEDs is dependent upon kinds of the semiconductor materials used in the manufacture of the LEDs. This is because the wavelength of emitted light is dependent upon the band-gap of the semiconductor materials representing an energy difference between valence-band electrons and conduction band electrons.
Gallium nitride (GaN) has a high thermal stability and a broad band gap (ranging from 0.8 to 6.2 eV), and therefore has received a great deal of attention in fields for the development of high-power output electronic devices. One of the reasons why the gallium nitride has attracted a great deal of interest is because it is possible to fabricate semiconductor layers emitting green, blue and white light, by using GaN in combination with other elements such as indium (In), aluminum (Al) and the like.
Due to the capability to control an emitted wavelength via the use of GaN, the emitted wavelength may be adjusted to a desired range suited for intrinsic properties of the materials used in compliance with the characteristics of specific equipment. For example, the use of GaN makes it possible to manufacture blue LEDs which are beneficial for optical writing and white LEDs which are capable of substituting incandescent lamps.
Green LEDs initially used GaP, which is an indirectly transitional material and has a low efficiency, and thus cannot emit pure green light. However, a green LED having high brightness was achieved by the success of the growth of an InGaN thin film.
Due to the above and other advantages of such GaN-based materials, a GaN-based LED market is rapidly growing. Accordingly, technologies of GaN-based optoelectronic devices have rapidly advanced from commercial introduction thereof in 1994.
As described above, nitride-based semiconductors, such as InGaN-based semiconductors, are directly transitional materials, thus being capable of forming LEDs having high brightness. However, due to high lattice mismatch and a difference of coefficients of thermal expansion with a different kind of substrate, the nitride-based semiconductors may have many defects in crystals. This is a problem awaiting solution.
That is, since substrates made of GaN-based materials cannot be easily manufactured and are expensive, when a light emitting device, such as an LED or an LD, is fabricated, a different kind of substrate, i.e., a sapphire substrate, is usually used.
However, when a GaN-based material is grown on the above substrate, defects, such as lattice mismatch or threading dislocation, may occur in the grown thin film due to a difference of coefficients of thermal expansion or a difference of lattice constants of crystal.
Accordingly, in order to fabricate a high-quality GaN-based LED, a GaN substrate having an excellent crystal structure is required.
Recently, a technique, which makes a freestanding GaN substrate using a hydride vapor phase epitaxy (HVPE) method, is being commercially used. However, such a technique still has many problems, such as bowing of the substrate and high production costs.
As a result, in order to fabricate an LED having the above structure, techniques, which reduce lattice defects or crystal defects, have been required.